Interactive video systems are being developed in which a master station can be accessed by a subscriber, using either a telephone line or a return channel in a duplex fibre optic video transmission link, whereby the subscriber can in the course of transmission of video material from the master station transmit back to the master station signals which will modify subsequently transmitted video material. British patent application GB 2120507A describes a video game system in which the head end of a cable TV system transmits on request an interactive video game programme. At various times during the course of the programme the subscriber may transmit instructions back to the head end to alter the subsequent playing of the game. Further descriptions of interactive video systems are given by Deloddere et al. in IEEE Communications Magazine vol. 32 No. 5 pp 82 to 88, and by Calvert et al. in Electrical Communication No. 3 September 94 pp 281 to 289.
With the development of ever more sophisticated video games requiring greater skill and speedier reflexes on the part of the subscriber a problem can arise whereby the propagation delays in both the transmission from the head end of the video game material and the transmission back to the head end of the subscriber's instructions can lead to incorrect modification of the subsequently transmitted video game material. Take for example a game in which the subscriber `player` is required to `shoot` a gun at a swiftly moving target. Such an action can require split-second timing. If the transmission propagation times are comparatively long the player's `shoot` instruction will be received at the head end some time after the target has effectively moved out of the `line-of-fire`. Of course, if for a particular subscriber the round trip propagation time is known then a pre-calculated adjustment factor can be built into the system. However, with modern cable TV systems a single head end can serve a large subscriber area with differing propagation times for all the subscribers.